The inclusion of an access door at the base of a metallic wind turbine tower entails a reduction in the cross-section of the tower wall that weakens said tower base. This effect is even more critical due to the fact that it is in the tower base where the loads are highest. Additionally, the access orifice wherein the door is installed causes a discontinuity in the tower that gives rise to the appearance of a stress concentration zone.
In order to mitigate the aforementioned adverse effects, the tower is usually reinforced in the door zone by a frame which is generally welded to the tower around the orifice.
In certain cases, the frame is obtained by cutting a plate, for example rectangular, having a thickness substantially greater than the thickness of the tower wall, as can be observed in FIGS. 1a and 1b. In said figures, an orifice (1) can be observed at the base (2) of a hollow metallic wind turbine tower, wherein a flat frame (3) is installed in the orifice (1), as described previously.
In order for the frame to be securely joined to the tower, the thickness of the tower wall in the orifice zone must be fully contained in the thickness of the frame along the entire perimeter of the orifice.
In embodiments of the state of the art, the frame is obtained by cutting a single flat piece. When the tower cross-section is curved, in order to securely join it to the tower and to ensure that the desired resistance is obtained, the thickness of the frame is greater than would be necessary if the frame had a curvature to conform to the curvature of the tower. Consequently, the frames are heavier and more expensive.
German patent DE102012019595 (e.n.o. Energy Systems) discloses a reinforcement frame for an opening located at the base of a wind turbine tower, wherein the frame is formed by two portions of laminated or forged steel, welded to the upper and lower part. The frame additionally comprises a welding lip for joining to the tower. The joint between the two parts that make up the frame is executed on the same plane, therefore the frame does not conform to the curvature of the tower. Consequently and as can be observed in the figures, the thickness of the frame is oversized.
Likewise, Chinese utility model CN201786578U (Shanghai Thaisheng Power Engineering Machinery) relates to a door frame, for a tower, said frame being formed from a plurality of parts joined therebetween, wherein at least an upper and lower part have a curvature in two directions to better conform to the curvature of the tower. Curving these parts in two directions is complex and expensive. In addition, this double curvature complicates the joint with the straight parts that form the frame, since the cross-sections of the two parts must coincide.